Detector configurations employing a scintillator crystal block optically coupled to an array of silicon photomultiplier sensor pads have wide applications both in particle physics laboratories as well as in detectors for medical, military and security purposes. Incident particles generate one or many photons in the scintillator crystal. The photons travel through the scintillator crystal until they hit an array of semiconductor photomultiplier sensor pads that is positioned at the surface of the crystal. The Silicon photomultiplier sensor pads typically comprise a plurality of avalanche photodiodes on a common silicon substrate. The dimensions of each single avalanche photodiode can vary from 20 to 100 μm, and their density can amount to 1000/mm2 or even more. Each avalanche photodiode in the photomultiplier sensor pad operates in a Geiger-mode and may be coupled to the adjacent photodiodes by means of a polysilicon quenching resistor. A reverse bias voltage (typically in the range of 20V to 100V in silicon) may be applied to each of the avalanche photodiodes, resulting in a gain between 105 and 106. By means of the photoelectric effect, a photon impinging on an avalanche photodiode will create a primary electron in the semiconductor substrate, which will then be amplified into an avalanche of electrons that generates a charge signal that can be read out and analyzed.
A charged particle traversing the crystal can generate a pulse of light by exciting the scintillator or by Cherenkov radiation. This light pulse is the origin of the photons that enter the avalanche photodiode. An energetic charged particle will generate a light pulse with an intensity that depends on its energy, while a gamma can undergo conversion and release an electron with an energy related to the energy of the initial gamma. A more intense light pulse from the scintillator will trigger more of the avalanche photodiodes and thus generate a larger signal. Thus by analysing the signal produced by a collection of these photodiodes, information concerning the timing and amplitude of the light pulse can be extracted.
For instance, detectors of this type may be used for time-of-flight measurements of incident particles that allow the deduction of the velocity of the incident particle. When combined with information on the curvature of the track of the particle in a magnetic field, from which the momentum of the particle can be deduced, the velocity information allows a determination of the mass of the particle. The detector configuration may also be employed as a highly sensitive gamma detector in medical applications, such as for positron emission tomography (PET).
The precision of the timing enhances the value of the measurement. This is clear for the time-of-flight application since more precise time information allow a better mass determination. For medical applications such as PET, precise time information results in clearer images and a reduced dose of radioactive tracers given to the patients.
A gamma detector based on Geiger mode avalanche photodiodes for applications in positron emission tomography is described in international patent application WO2012/152 587 A2. The silicon photomultipliers are arranged in strips that extend along the length of the edge of the scintillator crystal.
FIG. 1a is a schematic perspective view of another conventional detector configuration. The conventional detector configuration 100 comprises a scintillator crystal block 102 that is formed of a plurality of individual elongated scintillator elements 104. The detector configuration 100 further comprises a sensor array 106 that comprises a plurality of silicon photomultiplier sensor pads 108. The pads 108 are quadratic, and their dimensions correspond to the dimensions of the respective end surfaces of the scintillator elements 104. For illustrative purposes, FIG. 1a shows the sensor array 106 detached from the scintillator crystal block 102. However, for operation the sensor array 106 will be mounted to the end surface 110 of the scintillator crystal block 102 so that the sensor pads 108 lie on and correspond to the end surfaces of the scintillator elements 104. This establishes an optical coupling between the scintillator elements 104 and the sensor pads 108, and allows the sensor pads 108 to detect photons generated in the scintillator elements 104. Each of the sensor pads 108 is a silicon photomultiplier pad as described above, and is electrically coupled to a readout means (not shown in FIG. 1a) to detect the signal generated by the incident photons.
A second corresponding sensor array (not shown in FIG. 1a) may be placed on the end surface opposite from the surface 110 to increase the spatial and time resolution.
A conventional readout scheme for the sensor array 106 is shown in the schematic cross sectional view of FIG. 1b. FIG. 1b shows a cutout of one row of the sensor array 106 with five sensor pads 108a to 108e. The sensor pads 108a to 108e are connected to a common electrode 112 that electrically couples the sensor pads 108a to 108e. The common electrode 112 may be a common anode, but may also be a common cathode. As shown in FIG. 1b, the common electrode may be coupled to ground. The detector configuration 100 further comprises a plurality of individual electrodes 114a to 114e coupled to the respective sensor pads 108a to 108e. The electrodes 114a to 114e are separated from one another and are not directly electrically coupled with one another. The electrodes 114a to 114e are cathodes if the common electrode 112 is an anode, and vice versa. Each of the electrodes 114a to 114e is coupled to a respective plurality of corresponding front-end amplifiers 116a to 116e via respective electrical connections 118a to 118e. Each of the front-end amplifiers 116a to 116e may be coupled to a corresponding discriminator (not shown) for subsequent signal analysis. The front-end amplifiers 116a to 116e are coupled to ground via a common electrical link connection 120.
If one of the sensor pads 108a to 108e fires, such as the sensor pad 108b in FIG. 1b, the generated current flows through the electrical connection 118b and the input stage of the front end amplifier 116b and via the link connection 120 into the ground. Hence, the front end amplifier 116b and discriminator measure the current flowing through it with respect to ground. This ground may have many current spikes fed into it from other channels, which may lead to cross talk and timing jitter.
In their research article “Time based readout of a silicon photomultiplier (SiPM) for Time of Flight Positron Emission Tomography (TOF-PET)”, IEEE Transactions on Nuclear Science, Vol 58, No. 3, June 2011, P. Jarron et al. explain that the timing resolution may be significantly improved by replacing the common ground connection of FIG. 1b with a differential readout, wherein each of the sensor pads 108a to 108e has its own separate anode and cathode, which are both connected to the inputs of a corresponding differential current mode amplifier stage. A differential input connection has the advantage of superior rejection of ground and supply-voltage noise, which is a key feature for a fast multi-channel readout. However, for an array of SiPM detectors, the differential connection of each of the pads is complex and expensive. In addition, the electrical connections of the individual sensor pads lead to a dead area around each pad. These dead areas effectively reduce the efficiency of the photodetector.
What is required is a detector configuration that combines an improved timing with a high efficiency at the same time reducing the manufacturing complexity